MySQL---进阶查询

MySQL进阶查询

按关键字排序

• 使用ORDER BY语句来实现排序
• 排序可针对一个或多个字段
• ASC: 升序，默认排序方式
• DESC:降序

ORDER BY的语法结构

SELECT column1, column2, .. FROM table_ name ORDER BY column1, column2, ..
ASC|DESC;

按单字段排序

我创建了一个mask库，mess表

mysql> select * from mess;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | zs   |    90 |
|  2 | ls   |    80 |
|  3 | ww   |    70 |
|  4 | zl   |    60 |
+----+------+-------+
4 rows in set (0.00 sec)
###单字段降序
mysql> select * from mess where score>=70 order by score desc;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | zs   |    90 |
|  2 | ls   |    80 |
|  3 | ww   |    70 |
+----+------+-------+
3 rows in set (0.00 sec)
###单字段升序，默认即为升序ASC
mysql> select * from mess where score>=70 order by score;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  3 | ww   |    70 |
|  2 | ls   |    80 |
|  1 | zs   |    90 |
+----+------+-------+
3 rows in set (0.00 sec)

按多字段排序

多字段排序，大概就是先按一个字段进行排序，如果有相同的属性的值再对此进行排序；
如下先按分数进行升序，然后在分数相同的记录中再按照id进行升序排序

mysql> select * from mess order by score ,id ;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  4 | zl   |    60 |
|  3 | ww   |    70 |
|  5 | hb   |    70 |
|  2 | ls   |    80 |
|  1 | zs   |    90 |
+----+------+-------+
5 rows in set (0.00 sec)

对结果进行分组

• 使用GROUP BY语句来实现分组
• 通常结合聚合函数一起使用
• 可以按一个或多个字段对结果进行分组

GROUP BY的语法结构

SELECT column name, aggregate_ function(column_ name) FROM table_ name
WHERE column_ name operator value GROUP BY column_ name;

group by大概就是将字段进行分组，可以再对组内的数据进行统计
如下，把score进行分组，分完后计算组内name的个数；将分数进行降序排序

mysql> select count(name),score from mess group by score order by score desc;
+-------------+-------+
| count(name) | score |
+-------------+-------+
|           1 |    90 |
|           1 |    80 |
|           2 |    70 |
|           1 |    60 |
+-------------+-------+
4 rows in set (0.00 sec)
##group by 不与聚合函数用，可以将相应重复的字段给过滤了
mysql> select * from mess group by score order by score desc;
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  2 | ls   |    80 |     1 |
|  3 | ww   |    70 |     2 |
|  4 | zl   |    60 |     2 |
+----+------+-------+-------+

上面的语句中有一个count()聚合函数，可以有如下类型

• count(字段名称) 计数
• sum(字段名称) 求和
• avg(字段名称) 平均值
• max(字段名称) 最大值
• min(字段名称) 最小值

mysql> select avg(score) from mess;
+------------+
| avg(score) |
+------------+
|    74.0000 |
+------------+
1 row in set (0.00 sec)

mysql> select sum(score) from mess;
+------------+
| sum(score) |
+------------+
|        370 |
+------------+
1 row in set (0.00 sec)

mysql> select max(score) from mess;
+------------+
| max(score) |
+------------+
|         90 |
+------------+
1 row in set (0.00 sec)

限制结果条目

• 只返回SELECT查询结果的第一行或前几行
• 使用LIMIT语句限制条目

LIMIT语法结构

offset位置偏移量，从0开始
SELECT column1, column2, .. FROM table_ name LIMIT [offset,] number;
number返回记录行的最大数目

limit限制结果条目
显示3条

mysql> select * from mess limit 3 ;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | zs   |    90 |
|  2 | ls   |    80 |
|  3 | ww   |    70 |
+----+------+-------+
3 rows in set (0.00 sec)

不从第一条开始，从索引值后输出相应数量的条目，索引值从0开始
下面表示从索引值为1的记录开始，输出3条记录，也就是第2条到第5条记录

mysql> select * from mess limit 1,3;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  2 | ls   |    80 |
|  3 | ww   |    70 |
|  4 | zl   |    60 |
+----+------+-------+
3 rows in set (0.01 sec)

设置别名

• 使用AS语句设置别名，关键字AS可省略
• 设置别名时，保证不能与库中其他表或字段名称冲突

别名的语法结构

alias _name 列的别名
SELECT column_ name AS alias _name FROM table_ name;
alias name表的别名
SELECT column_ name(s) FROM table_ name AS alias name;
原字段或者表名在库内并不会被改变

字段别名

mysql> select count(*) from mess;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)
##别名
mysql> select count(*) as number from mess;
+--------+
| number |
+--------+
|      5 |
+--------+
1 row in set (0.00 sec)

表的别名

mysql> select a.id,a.name from mess as a;
+----+------+
| id | name |
+----+------+
|  1 | zs   |
|  2 | ls   |
|  3 | ww   |
|  4 | zl   |
|  5 | hb   |
+----+------+
5 rows in set (0.00 sec)

拓展
改善下原来的mess表

##插入一列award
mysql> alter table mess add column award int(3);
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from mess;
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |  NULL |
|  2 | ls   |    80 |  NULL |
|  3 | ww   |    70 |  NULL |
|  4 | zl   |    60 |  NULL |
|  5 | hb   |    70 |  NULL |
+----+------+-------+-------+
5 rows in set (0.00 sec)
##更新分数大于75的记录中award字段为1
mysql> update mess set award=1 where score>75;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from mess;
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  2 | ls   |    80 |     1 |
|  3 | ww   |    70 |  NULL |
|  4 | zl   |    60 |  NULL |
|  5 | hb   |    70 |  NULL |
+----+------+-------+-------+
5 rows in set (0.00 sec)
##更新分数小于75的记录中award字段为2
mysql> update mess set award=2 where score<75;
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select * from mess;
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  2 | ls   |    80 |     1 |
|  3 | ww   |    70 |     2 |
|  4 | zl   |    60 |     2 |
|  5 | hb   |    70 |     2 |
+----+------+-------+-------+
5 rows in set (0.00 sec)
mysql> create table awards (id int(3) primary key not null,awardname varchar(10) not null);
mysql> insert into awards values (1,'pen'),(2,'book');
mysql> select * from awards;
+----+-----------+
| id | awardname |
+----+-----------+
|  1 | pen       |
|  2 | book      |
+----+-----------+
##两表联合查询，mess表的award字段和awards表的id字段相关联
mysql> select * from mess inner join awards on mess.award=awards.id;
+----+------+-------+-------+----+-----------+
| id | name | score | award | id | awardname |
+----+------+-------+-------+----+-----------+
|  1 | zs   |    90 |     1 |  1 | pen       |
|  2 | ls   |    80 |     1 |  1 | pen       |
|  3 | ww   |    70 |     2 |  2 | book      |
|  4 | zl   |    60 |     2 |  2 | book      |
|  5 | hb   |    70 |     2 |  2 | book      |
+----+------+-------+-------+----+-----------+
5 rows in set (0.00 sec)
##只输出部分字段
mysql> select mess.name,awards.awardname from mess inner join awards on mess.award=awards.id;
+------+-----------+
| name | awardname |
+------+-----------+
| zs   | pen       |
| ls   | pen       |
| ww   | book      |
| zl   | book      |
| hb   | book      |
+------+-----------+
5 rows in set (0.00 sec)
###给表设置别名，as是可以省略的
mysql> select a.name,b.awardname from mess as a inner join awards b on a.award=b.id;
+------+-----------+
| name | awardname |
+------+-----------+
| zs   | pen       |
| ls   | pen       |
| ww   | book      |
| zl   | book      |
| hb   | book      |
+------+-----------+
5 rows in set (0.01 sec)
##给字段设置别名，
mysql> select a.name 姓名,b.awardname 奖励 from mess a inner join awards b on a.award=b.id;
+--------+--------+
| 姓名   | 奖励   |
+--------+--------+
| zs     | pen    |
| ls     | pen    |
| ww     | book   |
| zl     | book   |
| hb     | book   |
+--------+--------+
5 rows in set (0.00 sec)

通配符

通配符

• 用于替换字符串中的部分字符
• 通常配合LIKE一起使用，并协同WHERE完成查询

常用通配符

• %表示零个、一个或多个
• _表示单个字符

mysql> select * from mess where name like '%s';
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  2 | ls   |    80 |     1 |
+----+------+-------+-------+
2 rows in set (0.00 sec)

mysql> select * from mess where name like 'z_';
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  4 | zl   |    60 |     2 |
+----+------+-------+-------+
2 rows in set (0.00 sec)

子查询

子查询

• 也称作内查询或者嵌套查询
• 先于主查询被执行，其结果将作为外层主查询的条件
• 在增删改查中都可以使用子查询
• 支持多层嵌套
• IN语句是用来判断某个值是否在给定的结果集中

后面select语句结果是2，所以整个语句会输出mess表中award字段为2的记录
mysql>  select * from mess where award in (select id from awards where awardname='book');
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  3 | ww   |    70 |     2 |
|  4 | zl   |    60 |     2 |
|  5 | hb   |    70 |     2 |
+----+------+-------+-------+
3 rows in set (0.00 sec)
###注意使用的=号
mysql> select * from mess where award=(select id from awards where awardname='pen');
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  2 | ls   |    80 |     1 |
+----+------+-------+-------+
2 rows in set (0.00 sec)
##！=号
mysql> select * from mess where award!=(select id from awards where awardname='pen');
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  3 | ww   |    70 |     2 |
|  4 | zl   |    60 |     2 |
|  5 | hb   |    70 |     2 |
+----+------+-------+-------+
3 rows in set (0.00 sec)

exists,判断后面语句结果是否存在，存在则正确执行前面的语句，否则不能正确执行；exists后一般更select语句，跟DML语句insert会报错

mysql> select count(*) as number from mess where exists (select id from awards where id=1);
+--------+
| number |
+--------+
|      5 |
+--------+
1 row in set (0.00 sec)
##awards表中没有id为3的记录
mysql> select count(*) as number from mess where exists (select id from awards where id=3);
+--------+
| number |
+--------+
|      0 |
+--------+
1 row in set (0.00 sec)

NULL值

• 表示缺失的值
• 与数字0或者空白(spaces）是不同的
• 使用IS NULL或IS NOT NULL进行判断

NULL值和空值的区别

• 空值长度为0，不占空间;NULL值的长度为NULL，占用空间
• lS NULL无法判断空值
• 空值使用“="或者“<>"来处理
• COUNT()计算时，NULL会忽略，空值会加入计算

mysql> create table a (id int(3) primary key not null,name varchar(10));
mysql> insert into a values (1,'a');
mysql> insert into a values (2,'');
mysql> insert into a (id) values(3);
mysql> select * from a;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 |      |
|  3 | NULL |
+----+------+
3 rows in set (0.00 sec)
####COUNT()计算时，NULL会忽略，空值会加入计算
mysql> select count(name) from a;
+-------------+
| count(name) |
+-------------+
|           2 |
+-------------+
1 row in set (0.01 sec)

mysql> select * from a where name is not null;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 |      |
+----+------+
2 rows in set (0.00 sec)

正则表达式

• 根据指定的匹配模式匹配记录中符合要求的特殊字符
• 使用REGEXP关键字指定匹配模式

常用匹配模式

• p1|p2匹配p1或p2
• ^匹配开始字符
• […]匹配字符集中的任意一个字符，如[abc]匹配abc中的任意一个字符
• $匹配结束字符
• .匹配任意单个字符[^…]匹配不在中括号内的任何字符
• *匹配任意个前面的字符
• ＋匹配前面字符至少1次
• {n,m}匹配前面的字符串至少n次，至多m次
• {n}匹配前面的字符串n次

mysql> select * from awards
    -> ;
+----+-----------+
| id | awardname |
+----+-----------+
|  1 | pen       |
|  2 | book      |
|  3 | fruit     |
|  4 | car       |
|  5 | phone     |
+----+-----------+
##查找以name中以p开头的记录
mysql> select * from awards where awardname regexp '^p';
+----+-----------+
| id | awardname |
+----+-----------+
|  1 | pen       |
|  5 | phone     |
+----+-----------+
2 rows in set (0.00 sec)
###查找包含相应字符的记录，应满足字符连续，如搜索’pe’的时候phone就不会被查出来
mysql> select * from  awards where awardname regexp 'on';
+----+-----------+
| id | awardname |
+----+-----------+
|  5 | phone     |
+----+-----------+
1 row in set (0.00 sec)

mysql> select * from  awards where awardname regexp 'pe';
+----+-----------+
| id | awardname |
+----+-----------+
|  1 | pen       |
+----+-----------+
##‘*’的用法是表示‘*’前面的字符出现任意次，单独出现时区别于不在正则中的表示所有，会报错
mysql> select * from awards where awardname regexp '*';
ERROR 1139 (42000): Got error 'repetition-operator operand invalid' from regexp
mysql> select * from awards where awardname regexp '.*';
+----+-----------+
| id | awardname |
+----+-----------+
|  1 | pen       |
|  2 | book      |
|  3 | fruit     |
|  4 | car       |
|  5 | phone     |
+----+-----------+
5 rows in set (0.00 sec)

运算符

算术运算符
MySQL支持的算术运算符

• ＋加法
• • 减法
• *乘法
• /除法
• %取余数

mysql> select 1+2,2-1,2*1,4/2,6%2;
+-----+-----+-----+--------+------+
| 1+2 | 2-1 | 2*1 | 4/2    | 6%2  |
+-----+-----+-----+--------+------+
|   3 |   1 |   2 | 2.0000 |    0 |
+-----+-----+-----+--------+------+
1 row in set (0.00 sec)

比较运算符

• 字符串的比较默认不区分大小写，可使用binary来区分
  常用比较运算符
• =等于

• 大于

• <小于

• =大于等于

• <=小于等于
• !=或<>不等于
• IN在集合中
• LIKE 通配符匹配
• lS NULL 判断─个值是否为NULL
• lS NOT NULL 判断—个值是否不为NULL
• BETWEEN AND两者之间
• GREATEST两个或多个参数时返回最大值
• LEAST 两个或多个参数时返回最小值

mysql> select 2=3,2='2','e'='e',(1+3)=(2+2),'a'=NULL;
+-----+-------+---------+-------------+----------+
| 2=3 | 2='2' | 'e'='e' | (1+3)=(2+2) | 'a'=NULL |
+-----+-------+---------+-------------+----------+
|   0 |     1 |       1 |           1 |     NULL |
+-----+-------+---------+-------------+----------+
1 row in set (0.00 sec)

从以上查询可以看出来：

• 如果两者都是整数，则按照整数值进行比较。
• 如果一个整数一个字符串，则会自动将字符串转换为数字，再进行比较。
• 如果两者都是字符串，则按照字符串进行比较。
• 如果两者中至少有一个值是 NULL，则比较的结果是 NULL。

mysql> select 'njit'<>'njiu',1<>1,3!=3,NULL<>NULL;
+----------------+------+------+------------+
| 'njit'<>'njiu' | 1<>1 | 3!=3 | NULL<>NULL |
+----------------+------+------+------------+
|              1 |    0 |    0 |       NULL |
+----------------+------+------+------------+
1 row in set (0.00 sec)

需要注意的是不等于运算符不能用于判断 NULL

mysql> select 5>4,'a'>'b',2>=3,(2+3)>=(1+2),4.4<3,1<2,'x'<='y',5<=5.5,'u'>=NULL;
+-----+---------+------+--------------+-------+-----+----------+--------+-----------+
| 5>4 | 'a'>'b' | 2>=3 | (2+3)>=(1+2) | 4.4<3 | 1<2 | 'x'<='y' | 5<=5.5 | 'u'>=NULL |
+-----+---------+------+--------------+-------+-----+----------+--------+-----------+
|	1 |	0 |	0 |	1 |	0 |	1 |	1 |	1 |	NULL |
+-----+---------+------+--------------+-------+-----+----------+--------+-----------+
1 row in set (0.00 sec)

大于、大于等于、小于等于都是不用于判断NULL
关于数字、字符和 NULL 值的运用如下
IS NULL 和 IS NOT NULL 一个判断为空，另一个判断不为空，只是有无 NOT 这个关键字的区别，同时返回值不同。

mysql> select 2 IS NULL,'f' IS NOT NULL,NULL IS NULL;
+-----------+-----------------+--------------+
| 2 IS NULL | 'f' IS NOT NULL | NULL IS NULL |
+-----------+-----------------+--------------+
|         0 |               1 |            1 |
+-----------+-----------------+--------------+
1 row in set (0.01 sec)

BETWEEN AND 比较运算通常用于判断一个值是否落在某两个值之间,如果是比较字母的话，会转换成ASCII码进行比较

mysql> select 4 BETWEEN 2 AND 4,5 BETWEEN 5 AND 6,'B' BETWEEN 'A' AND 'C';
+-------------------+-------------------+-------------------------+
| 4 BETWEEN 2 AND 4 | 5 BETWEEN 5 AND 6 | 'B' BETWEEN 'A' AND 'C' |
+-------------------+-------------------+-------------------------+
|                 1 |                 1 |                       1 |
+-------------------+-------------------+-------------------------+
1 row in set (0.00 sec)

LEAST：当有两个或者多个参数时，返回其中的最小值。如果其中一个值为 NULL，则返回结果就为 NULL。
GREATEST：当有两个或者多个参数时，返回其中的最大值。如果其中一个值为 NULL， 则返回结果就为 NULL。

mysql> select least(1,2,3),least('a','b','c'),greatest(1,2,3),greatest('a','b','c'),least(1,2,null);
+--------------+--------------------+-----------------+-----------------------+-----------------+
| least(1,2,3) | least('a','b','c') | greatest(1,2,3) | greatest('a','b','c') | least(1,2,null) |
+--------------+--------------------+-----------------+-----------------------+-----------------+
|            1 | a                  |               3 | c                     |            NULL |
+--------------+--------------------+-----------------+-----------------------+-----------------+
1 row in set (0.00 sec)

IN 判断一个值是否在对应的列表中，如果是返回 1，否则返回 0。
NOT IN 判断一个值是否不在对应的列表中，如果不是返回 1，否则返回 0。

mysql> select 2 in (1,2,3,4),'c' not in ('a','b','c');
+----------------+--------------------------+
| 2 in (1,2,3,4) | 'c' not in ('a','b','c') |
+----------------+--------------------------+
|              1 |                        0 |
+----------------+--------------------------+
1 row in set (0.00 sec)

LIKE 用来匹配字符串，如果匹配成功则返回 1，反之返回 0。LIKE 支持两种通配符：’%’ 用于匹配任意数目的字符，而’_’只能匹配一个字符。

mysql> select 'aug' like 'au_','njit' like 'n%','zn' not like 'z%';
+------------------+------------------+--------------------+
| 'aug' like 'au_' | 'njit' like 'n%' | 'zn' not like 'z%' |
+------------------+------------------+--------------------+
|                1 |                1 |                  0 |
+------------------+------------------+--------------------+
1 row in set (0.00 sec)

逻辑运算符

• 又被称为布尔运算符用来判断表达式的真假

常用的逻辑运算符

• NOT或!:逻辑非
• AND或&&:逻辑与
• OR或||:逻辑或
• XOR:罗辑异或

逻辑非，分为0值和非零值

mysql> select not 2,!3,not 0,!(4-4);
+-------+----+-------+--------+
| not 2 | !3 | not 0 | !(4-4) |
+-------+----+-------+--------+
|     0 |  0 |     1 |      1 |
+-------+----+-------+--------+
1 row in set (0.00 sec)

逻辑与,逻辑与通常用于判断两个值或多个值的有效性，如果所有值都是真返回 1，否则返回 0。

mysql> select 2 and 0,2 and 3,0 && null,1 && null;
+---------+---------+-----------+-----------+
| 2 and 0 | 2 and 3 | 0 && null | 1 && null |
+---------+---------+-----------+-----------+
|       0 |       1 |         0 |      NULL |
+---------+---------+-----------+-----------+
1 row in set (0.00 sec)

逻辑或,逻辑或表示包含的操作数，任意一个为非零值并且不是 NULL 值时，返回 1，否则返回0。还是使用or比较好，使用||显示结果好像会有错误

mysql> select 2 or 0,2 or 3,0 or null,1 or null;
+--------+--------+-----------+-----------+
| 2 or 0 | 2 or 3 | 0 or null | 1 or null |
+--------+--------+-----------+-----------+
|      1 |      1 |      NULL |         1 |
+--------+--------+-----------+-----------+
1 row in set (0.00 sec)

逻辑异或,两个非 NULL 值的操作数，如果两者都是 0 或者都是非 0，则返回 0；如果一个为 0， 另一个为非 0，则返回结果为 1；当任意一个值为 NULL 时，返回值为 NULL

mysql> select 1 xor 1,0 xor 1,0 xor null,1 xor null;
+---------+---------+------------+------------+
| 1 xor 1 | 0 xor 1 | 0 xor null | 1 xor null |
+---------+---------+------------+------------+
|       0 |       1 |       NULL |       NULL |
+---------+---------+------------+------------+
1 row in set (0.00 sec)

位运算符

• 位运算符实际上是对二进制数进行计算的运算符
  位运算符
• &：按位与
• |：按位或
• ~：按位取反
• ^：按位异或
• <<:按位左移

• :按位右移

将十进制数转换成二进制数然后进行相应的位运算，5 & _{1大概就是5为,0101,1为0001，}1为1110，把0101和1110进行与运算。

mysql> select 10 & 15,10 | 15,10 ^ 15,5 & ~1;
+---------+---------+---------+--------+
| 10 & 15 | 10 | 15 | 10 ^ 15 | 5 & ~1 |
+---------+---------+---------+--------+
|      10 |      15 |       5 |      4 |
+---------+---------+---------+--------+
1 row in set (0.01 sec)

左移右移运算符
1为0001向左移两位的话就成了0100结果为4,5为0101向左移一位的话1010结果为10,
5为0101，向右移两位的话就成了0001结果为1

mysql> select 1<<2;
+------+
| 1<<2 |
+------+
|    4 |
+------+
1 row in set (0.00 sec)

mysql> select 5<<1;
+------+
| 5<<1 |
+------+
|   10 |
+------+
1 row in set (0.00 sec)

mysql> select 5>>2;
+------+
| 5>>2 |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

运算符优先级
优先级 运算符 优先级 运算符
1 ！ 8 |
2 ~ 9 =,<=>,>=,>,<=,<,<>,!=,IS,LIKE,REGEXP,IN
3 ^ 10 BETWEEN,CASE,WHEN,THEN,ELSE
4 * , /(DIV), %(MOD) 11 NOT
5 +,- 12 &&,AND
6 >>,<< 13 ||,OR,XOR
7 & 14 :=

连接查询

MySQL 的连接查询，通常都是将来自两个或多个表的行结合起来，基于这些表之间的共同字段，进行数据的拼接。首先，要确定一个主表作为结果集，然后将其他表的行有选择 性的连接到选定的主表结果集上。使用较多的连接查询包括：内连接、左连接和右连接。
内连接inter join ,会显示两张连接表的交集

mysql> select * from mess;
+----+------+-------+-------+
| id | name | score | award |
+----+------+-------+-------+
|  1 | zs   |    90 |     1 |
|  2 | ls   |    80 |     1 |
|  3 | ww   |    70 |     2 |
|  4 | zl   |    60 |     2 |
|  5 | hb   |    70 |     2 |
|  6 | ad   |    60 |     0 |
+----+------+-------+-------+
6 rows in set (0.00 sec)

mysql> select * from awards;
+----+-----------+
| id | awardname |
+----+-----------+
|  1 | pen       |
|  2 | book      |
|  3 | fruit     |
|  4 | car       |
|  5 | phone     |
+----+-----------+

mysql> select * from mess inner join awards on mess.award=awards.id;
+----+------+-------+-------+----+-----------+
| id | name | score | award | id | awardname |
+----+------+-------+-------+----+-----------+
|  1 | zs   |    90 |     1 |  1 | pen       |
|  2 | ls   |    80 |     1 |  1 | pen       |
|  3 | ww   |    70 |     2 |  2 | book      |
|  4 | zl   |    60 |     2 |  2 | book      |
|  5 | hb   |    70 |     2 |  2 | book      |
+----+------+-------+-------+----+-----------+
5 rows in set (0.00 sec)
mysql> select mess.name,mess.score,mess.award from mess inner join awards on mess.award=awards.id;
+------+-------+-------+
| name | score | award |
+------+-------+-------+
| zs   |    90 |     1 |
| ls   |    80 |     1 |
| ww   |    70 |     2 |
| zl   |    60 |     2 |
| hb   |    70 |     2 |
+------+-------+-------+
5 rows in set (0.00 sec)

外连接
左连接，将左边的表作为主表，右边作为匹配相应字段的表，主表将显示所有

mysql> select * from mess left join awards on mess.award=awards.id;
+----+------+-------+-------+------+-----------+
| id | name | score | award | id   | awardname |
+----+------+-------+-------+------+-----------+
|  1 | zs   |    90 |     1 |    1 | pen       |
|  2 | ls   |    80 |     1 |    1 | pen       |
|  3 | ww   |    70 |     2 |    2 | book      |
|  4 | zl   |    60 |     2 |    2 | book      |
|  5 | hb   |    70 |     2 |    2 | book      |
|  6 | ad   |    60 |     0 | NULL | NULL      |
+----+------+-------+-------+------+-----------+

右连接,右边的表作为基础表，会显示所有表的内容，左边的表匹配上右边表的也会显示

mysql> select * from mess right join awards on mess.award=awards.id;
+------+------+-------+-------+----+-----------+
| id   | name | score | award | id | awardname |
+------+------+-------+-------+----+-----------+
|    1 | zs   |    90 |     1 |  1 | pen       |
|    2 | ls   |    80 |     1 |  1 | pen       |
|    3 | ww   |    70 |     2 |  2 | book      |
|    4 | zl   |    60 |     2 |  2 | book      |
|    5 | hb   |    70 |     2 |  2 | book      |
| NULL | NULL |  NULL |  NULL |  3 | fruit     |
| NULL | NULL |  NULL |  NULL |  4 | car       |
| NULL | NULL |  NULL |  NULL |  5 | phone     |
+------+------+-------+-------+----+-----------+
8 rows in set (0.00 sec)